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The Bureau of Mines is developing melting data for binary combinations of the alkaline earth fluorides and yttria to facilitate the formulation of slag compositions. The present report deals with the caf2-y2o3 system for which solidus and liquidus temperatures and solid solubilities were determined. The information is presented as the phase diagram. All specimen compositions were blended from high-purity materials and encapsulated in mo or pt20rh prior to heat treatment. Liquidus temperatures were determined by ceramographic examination of heat- treated and quenched specimens. Solidus temperatures were determined by differential thermal analysis and by electron microprobe analysis of quenched specimens. Terminal solid solubilities were derived by parametric methods. The peritectic system was found to be pseudobinary due to the formation of yttrium oxyfluoride at temperatures near the liquidus. The peritectic point occurs at 7 mole percent y2o3 and 1,438 deg. +-5 Deg. C. The terminal solid solubility of y2o3 in caf2 decreases from 7 mole percent at 1,438 deg. C to 1 mole percent at 1,100 deg. C, and the solubility of caf2 in y2o3 decreases from 9.5 to 7 mole percent over the same temperature interval.